A period is the part of the menstrual cycle when a woman bleeds from her vagina for a few days. In most women this happens every 28 days or so.
It's common for women to have a cycle that's longer or shorter than this, from 24 to 35 days.
Girls have their first period when they start puberty. This can be between the ages of eight and 16 years, although girls most commonly start their periods at about 12 years old. Your periods will continue until the menopause, which usually occurs at the age of around 45 to 55.
The menstrual cycle
Each menstrual cycle starts on the first day of your period (day one) and lasts until the day before your next period begins.
To understand periods and the menstrual cycle, it helps to know about the reproductive organs inside a woman's body. These are:
- two ovaries, where eggs are stored, developed and released
- the womb (uterus), where a fertilised egg implants and a pregnancy grows
- two thin tubes called fallopian tubes, which connect the ovaries to the womb
- the cervix, which is the lower part of the womb that connects to the vagina
- the vagina, which is a tube of muscle connecting the cervix to the outside of the body
In each menstrual cycle, rising levels of the hormone oestrogen cause the ovary to develop an egg and release it (this process is called ovulation). The womb lining gets thicker, ready for a possible pregnancy.
The egg travels down the fallopian tube and you can get pregnant now if the egg meets any sperm. Find out more about your fertile time and getting pregnant.
The egg lives for about 24 hours. If pregnancy doesn't happen, the egg is absorbed into your body. The womb lining comes away and leaves the body, mixed with blood, as a period (the menstrual flow).
Getting pregnant
Your fertile time (when you can get pregnant) can be difficult to pinpoint. It's around the time you ovulate, which in most women is around 12-14 days before the start of their next period.
A woman can't get pregnant if she doesn't ovulate. Some hormonal methods of contraception, such as the combined pill, the patch and the injection work by stopping ovulation.
Body changes
Your body produces different amounts of hormones at different times during your cycle. This can cause changes in your body and your emotions.
Your vaginal secretions, for example, change throughout your menstrual cycle. Around the time of ovulation they become thinner and stretchy, a bit like raw egg white.
You may also have mood swings in the days before your period, or notice your breasts getting swollen and painful.
Premenstrual syndrome (PMS) is a range of symptoms that can affect women in the days before their period.
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